1. Field of the Invention
The present invention relates to shading correction in an image reading apparatus, and particularly to a technique for realizing acquisition of a proper white reference value in shading correction.
2. Description of the Related Art
Traditionally, in the processing to read a white reference board when performing shading correction in an image reading apparatus, several lines are read by a line sensor and the average value of these is used as a white reference value.
Also a technique of finding a white reference value by reading white reference data of several lines by a line sensor and then statistically processing the data, instead of simply averaging the data, is known (see JP-A-11-289432).
The traditional techniques are carried out to reduce the influence of foreign matter adhering to the white reference board in the case of performing shading correction in the image reading apparatus.
In the method using an average value of white reference data or the method using statistical processing as in the traditional techniques, in the case where detection, removal and correction of foreign matter on the white reference board is carried out, it is possible to find a white reference value with less adverse effects if the foreign matter adhering to the white reference board is reasonably small.
However, if the foreign matter adhering to the white reference board is too large to correct sufficiently, it affects the white reference value even when the processing as in the traditional techniques is carried out.